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    Navigation News article on leap seconds.
    From: George Huxtable
    Date: 2005 Oct 9, 15:13 +0100

    "Navigation News" is the chatty journal of the Royal Institute of
    Navigation, which appears in alternate months. Don't confuse it with their
    more formal and academic "The Journal of Navigation", which appears three
    times a year.
    
    The September / October issue of 2005, contains an article referring to the
    proposal, coming principally from the US military, that leap seconds in UTC
    should be discontinued from 2008, though it would continue to be called
    UTC. Those leap seconds ensure, at present, that UTC and GMT are never more
    than 0.9 seconds apart. The article explains the background, and the
    (opposing) reception of the Institute to the proposal.
    
    UTC is an accurate, and uniform, time scale based on atomic clocks. GMT is
    based on the uneven, and slowing, and unpredictable, rate of rotation of
    the Earth. Nobody, not even the US Military, can do anything about that
    slowing. The laws of physics, and for example the motion of planets in
    their orbits, depend on an unchanging measure of the passage of time, and
    are quite unaffected by what the speed of rotation of the third planet out
    from an undistinguished star in a not-very-special galaxy happens to be at
    that moment.
    
    In UTC, every second is exactly the same as any other second, so it's the
    unchanging measure of time that the laws of physics call for. However, as a
    compromise, the COUNTING of those seconds in terms of minutes, hours, days,
    is adjusted to keep approximately in step with the Earth's rotation. Every
    now and again since 1972 a leap-second jump has been made made in UTC to
    keep it in step with that slowing rate. Otherwise, UTC would gradually
    diverge, more and more, from mean time, at which the mean sun is overhead
    at noon, at Greenwich.
    
    Since 1978, a leap second has been inserted only about every year or two,
    and so far these have amounted to 22 seconds. Not a big deal, you might
    think, but you have to look at the future, when the slowing has progressed
    further. If the Earth keeps on slowing it its present rate (which is far
    from certain) then in 2078, without leap seconds, our clocks would be
    somewhere near 5 minutes out of step with the Sun, and by about 2300 would
    be an hour out. At the end of the millennium we would be taking lunch in
    the dark! Mind you, by that time, to keep in step with the Sun would
    require inserting leap-seconds once a week, which would present its own
    problems..
    
    To be responsible, we must ensure that the system that the world adopts for
    counting and measuring time works sensibly over long periods, and is not
    just a short-term fix.
    
    It seems that the US Military have not chosen to explain what their problem
    is, that dropping leap-seconds would fix for them. It's not as if there is
    no alternative.  Both International Atomic Time and GPS System time are
    existing time scales, which do not insert leap seconds. It's unclear why
    the US Military can't simply asdopt one of those time scales to overcome
    any present problem, without tinkering with the way the whole world sets
    its clocks.
    
    This US proposal comes up for decision at the ITU meeting next month. The
    RIN are advising the UK government to oppose it. No doubt there are
    differing views, and it would be interesting to learn if any Nav-l members
    can offer another viewpoint.
    
    George.
    ===============================================================
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.
    
    
    

       
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